Overriding files in GCS bucket

[amlanroy1980]

I am reading product content in JSON format from Kafka and creating files in the GCS bucket. The file content is JSON, one of the fields in the JSON is update time. I am creating one file for each product.

I want to make sure that I do not override the file if the data in the GCS bucket is more recent than the incoming data (based on update time).

I can read the file, parse the JSON, compare the update time, and then override the file. But I want to know if there is a more efficient approach.

[MaxImbrox]

Hi @amlanroy1980,

rather than reading the existing file, parsing its JSON, and comparing the update time, you can use metadata and conditions in GCS to perform this check directly when uploading a file:

1. Metadata Storage: Store the update time of each product file as metadata in GCS. Metadata can be set and retrieved easily without having to download and parse the JSON file.

2. Conditional Uploads: When uploading a file, you can use GCS's If-Match or If-None-Match headers to conditionally update files only if certain conditions are met. This can be based on the metadata you stored for the existing file.

3. Check Metadata Before Upload:

   • First, retrieve the metadata of the existing file.
   • Compare the update_time in the incoming data with the update_time stored in the metadata.
   • If the incoming data is more recent (i.e., has a later update_time), proceed with the file upload and update the metadata accordingly.

4. Optimized GCS Interactions:

   • Instead of reading the file and parsing it, you can directly retrieve the metadata from the existing file.
   • Using conditional operations reduces unnecessary file reads/writes.

To avoid the data overwriting you can in any case enable the object versioning to archive all versions of the same file.

[amlanroy1980]

Thank you, @MaxImbrox. I have a follow-up query.

I understand the approach where I am storing the update time in metadata, and then retrieving it to decide whether to override the file or not. I am assuming this would be done on the client side.

I did not understand how to use the If-Match or If-None-Match headers in the process. Could you share some more details about it?

[MaxImbrox · Accepted solution]

Hi again @amlanroy1980,

In Google Cloud Storage (GCS), conditional uploads allow you to upload files to the bucket only if certain conditions are met. These conditions can be based on the metadata of an existing file in the bucket, such as the etag (entity tag) or update time. By using conditional headers, you can control whether an upload should proceed based on the current state of the file in GCS.

Conditional Headers in GCS --> 

https://cloud.google.com/storage/docs/request-preconditions

GCS supports two conditional headers for uploads: If-Match:

1. • This header allows you to specify the etag of the existing file in the bucket.
   • The upload will proceed only if the existing file's etag matches the etag provided in the header.
   • This is useful for ensuring that you only update a file if it has not been modified since you last checked its etag.

2. If-None-Match:

   • This header allows you to specify the etag of the existing file in the bucket.
   • The upload will proceed only if the existing file's etag does not match the etag provided in the header.
   • This is useful for ensuring that you only upload a new file if the file does not currently exist in the bucket.

Here's an example of how you might use the If-Match header in Python using the Google Cloud Storage client library:

---------

from google.cloud import storage

# Initialize the GCS client
client = storage.Client()

# Define the bucket and file name
bucket_name = 'your-bucket-name'
file_name = 'your-file-name.json'

# Get the bucket
bucket = client.bucket(bucket_name)

# Get the existing file blob
blob = bucket.blob(file_name)

# Retrieve the metadata of the existing file
if blob.exists():
existing_metadata = blob.metadata
existing_etag = blob.etag

# Compare update time (assuming it's stored as a custom metadata field)
incoming_update_time = '2024-04-01T12:00:00Z' # Replace with the incoming update time

# Perform the conditional upload only if the incoming update time is more recent
if incoming_update_time > existing_metadata.get('update_time', ''):
# Set the If-Match header with the existing etag
blob.upload_from_filename('path/to/new/file.json', if_match=existing_etag)
print('File updated successfully.')
else:
print('No update needed; incoming data is not more recent.')
else:
# If the file does not exist, upload the new file without conditions
blob.upload_from_filename('path/to/new/file.json')
print('File uploaded successfully.')

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In this example, you first retrieve the metadata of the existing file and compare the update time with the incoming data. If the incoming data is more recent, you proceed with the conditional upload using the If-Match header. Otherwise, the upload is skipped.